Promtail-generated metrics have promtail_custom prefix

[AndrewCooper]

Describe the bug
Metrics created by the metrics pipeline stage have a prefix of "promtail_custom_". This is not documented anywhere and not logged so I eventually figured it out by scraping the metrics and browsing through them manually.

To Reproduce
Steps to reproduce the behavior:
Add a unique metric name to a scrape_config pipeline, like foo_my_metric.
Scrape the metrics manually.
See the metric promail_custom_foo_my_metric in the output.

Expected behavior
Either this behavior should be removed or should be documented.

Environment:
docker image: grafana/promtail:v1.3.0

[adityacs]

@cyriltovena Instead of removing the prefix, can we make it user defined like below?
prefix: user_custom

pipeline_stages:
- json:
    expressions:
      app: app
      payload: payload
- metrics:
    loki_count:
      type: Counter
      description: uhhhhhhh
      source: app
      prefix: user_custom
      config:
        value: loki
        action: inc